i buy my protection plans selectively and carefully. i buy them on sale in the case of squaretrade or in the case of applecare, on ebay. i also only buy them based on the liklihood of needing the repairs do to the abuse they will endure in my life and their liklihood of failure.
desktops - never, except on dell desktops for mission critical work pc's where their 4 hour on site is worth its weight in gold if you need it.
laptops- apple care always, and it has always paid off. for the new air i might reevaluate and consider squaretrade instead of applecare.
ipods - never. their cost to replace is low and their critical lifespan is relatively short.
phones - the potential for me to drop and break is high so yes to squaretrade. bad value if you compare it to the subsidized price, not so bad when you compare to the replacement cost you will pay. i guess that apple will sell you a refurb for less is a consideration i havent considered, but still, i almost always without exception have destroyed a phone a year. this is my first iphone and so far i only dropped it (almost daily) the first couple weeks i had it but since then never, so this one might outlast the warranty.
ipad - i carry it with me always and in a barely protective case, so the likelihood of damage is high. squaretrade.
cameras - sony usually has some sort of promotion for their extended care making it a no brainer since again, i am hard on my portable electronics and the likelihood of failure or damage is high.